Discussion of “correlations for fully softened shear strength parameters” by B. A. Castellanos, T. L. Brandon, and D. R. Vandenberge, this article was published in Geotechnical Testing Journal, Vol. 39, No. 4, 2016. [DOI: 10.1520/gtj20150184]

Castellanos and team of researchers have proposed an empirical correlation for fully softened shear strength based on data from drained direct shear tests on reconstituted specimens, with plasticity index in the range of 6 % to 63 %, and clay size fraction in the range of 10 % to 70 %. Fully softened shear strength condition may represent the limiting mobilized shear strength on all or part of the slip surface. In other words, fully softened shear strength condition may be a factor in stability of all stiff clays and clay shales with clay size fraction or plasticity index more than 20 %, and not all of these are ‘high-plasticity clays’. The applicability to field conditions of the Stark and Eid data on fully softened shear strength, expressed in terms of secant fully softened friction angle as a function of plasticity index and effective normal stress, has been confirmed by Mesri and Shahien by means of back-analyses of 40 first-time slope failures in stiff clays.